In the state-of-the-art commercial transport aircraft, there are normally three independent active operating hydraulic systems. The aircraft's primary flight critical control surfaces normally simultaneously utilize all three of these hydraulic systems. The reasoning behind the simultaneous use of the three independent hydraulic systems is that hydraulic systems in aircraft experience a relative high failure rate. It is well recognized as basic to safe aircraft operation that flight critical control surfaces be powered even though one or more of the hydraulic systems have experienced a failure and have been lost.
Normally aircraft primary flight control systems have two distinct modes of operation, namely, one of gross motion such as the rudder mode on an aircraft, and the other of trim motion or damping motion such as the yaw damping mode of the rudder. In typical prior art linear hydraulic servo cylinder systems, a summing bar is utilized to receive a mechanical input to accomplish the positioning of a main power valve of a servo cylinder which results in a gross motion actuation mode. The opposite end of the summing bar is typically provided with an electro-hydraulic servo valve input which provides a damping input to the aforementioned main power valve.
In these types of prior art systems, parallel actuators or tandem actuators (see R. Westbury, U.S. Pat. No. 2,597,430) are utilized to provide for redundancy. As noted earlier, this requires that all independent hydraulic systems be active at all times except in the event of a hydraulic system failure. The prior art arrangements potentially result in higher forces than required being imposed upon the flight control surface since each of the parallel or tandem actuators have the capacity for operating the flight control surface alone.
Since all independent hydraulic systems are active at all times, excess hydraulic power is consumed resulting in a lower overall efficiency of the aircraft prime mover (engine) or the auxiliary power unit.
It is therefore a basic objective of the invention to be described hereinafter to replace the prior art servo cylinder systems heretofore described with a hydromechanical system that will provide the same kind or better redundancy and failure protection in the event of various types of mechanical, hydraulic and signal failures.
The desirability of redundancy of the type described aboard an aircraft has been recognized in the patented art by such inventors as D. Wood et al in their U.S. Pat. No. 3,368,351 directed to a "Redundant Control System"; G. D. Jenney in his U.S. Pat. No. 3,496,836 directed to a "Redundant Control System having Fail-Operate, Fail-Neutral and Channel Emergency Select", as well as Koopman et al in their U.S. Pat. No. 3,877,346 directed to an "Electro-hydraulic Actuation System with Redundancy Operation, Supervision and Emergency Operation". These inventions, however, do not provide the inventive approach to be described more fully hereinafter wherein a rudder and trim control actuation system uses these hydraulic motors each operating from its own independent hydraulic system where two motors are active at all times and the third is a standby motor that will provide redundancy for both of the other two motors. Nor do these referenced inventions provide for an arrangement that entertains and copes with the event of a hydraulic system failure by having the stand-by motor coupled into the failed system to back-up the failed motor. In addition, the invention to be described also provides a novel arrangement to cope with the event that both of the first two motors should fail. Should this failure occur, the standby motor can be utilized to operate both the gross motion and damping motion described earlier.